Add documentation for consumption metrics

docs/consumption_metrics.md

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+### Overview
+Pageserver and proxy periodically collect consumption metrics and push them to a HTTP endpoint.
+
+- The metrics are collected in a separate thread, and the collection interval and endpoint are configurable.
+
+- Metrics are cached, so that we don't send unchanged metrics on every iteration.
+
+- Metrics are sent in batches of 1000 (see CHUNK_SIZE const) metrics max with no particular grouping guarantees.
+
+batch format is
+```json
+
+{ "events" : [metric1, metric2, ...]]}
+
+```
+See metric format examples below.
+
+- All metrics values are in bytes, unless otherwise specified.
+
+- Currently no retries are implemented.
+
+### Pageserver metrics
+
+#### Configuration
+The endpoint and the collection interval are specified in the pageserver config file (or can be passed as command line arguments):
+`metric_collection_endpoint` defaults to None, which means that metric collection is disabled by default.
+`metric_collection_interval` defaults to 10min
+
+#### Metrics
+
+Currently, the following metrics are collected:
+
+- `written_size`
+
+Amount of WAL produced , by a timeline, i.e. last_record_lsn
+This is an absolute, per-timeline metric.
+
+- `resident_size`
+
+Size of all the layer files in the tenant's directory on disk on the pageserver.
+This is an absolute, per-tenant metric.
+
+- `remote_storage_size`
+
+Size of the remote storage (S3) directory.
+This is an absolute, per-tenant metric.
+
+- `timeline_logical_size`
+Logical size of the data in the timeline
+This is an absolute, per-timeline metric.
+
+- `synthetic_storage_size`
+Size of all tenant's branches including WAL
+This is the same metric that `tenant/{tenant_id}/size` endpoint returns.
+This is an absolute, per-tenant metric.
+
+Synthetic storage size is calculated in a separate thread, so it might be slightly outdated.
+
+#### Format example
+
+```json
+{
+"metric": "remote_storage_size",
+"type": "absolute",
+"time": "2022-12-28T11:07:19.317310284Z",
+"idempotency_key": "2022-12-28 11:07:19.317310324 UTC-1-4019",
+"value": 12345454,
+"tenant_id": "5d07d9ce9237c4cd845ea7918c0afa7d",
+"timeline_id": "a03ebb4f5922a1c56ff7485cc8854143",
+}
+```
+
+`idempotency_key` is a unique key for each metric, so that we can deduplicate metrics.
+It is a combination of the time, node_id and a random number.
+
+### Proxy consumption metrics
+
+#### Configuration
+The endpoint and the collection interval can be passed as command line arguments for proxy:
+`metric_collection_endpoint` defaults to None, which means that metric collection is disabled by default.
+`metric_collection_interval` defaults to 10min
+
+#### Metrics
+
+Currently, only one proxy metric is collected:
+
+- `proxy_io_bytes_per_client`
+Outbound traffic per client.
+This is an incremental, per-endpoint metric.
+
+#### Format example
+
+```json
+{
+"metric": "proxy_io_bytes_per_client",
+"type": "incremental",
+"start_time": "2022-12-28T11:07:19.317310284Z",
+"stop_time": "2022-12-28T11:07:19.317310284Z",
+"idempotency_key": "2022-12-28 11:07:19.317310324 UTC-1-4019",
+"value": 12345454,
+"endpoint_id": "5d07d9ce9237c4cd845ea7918c0afa7d",
+}
+```
+
+The metric is incremental, so the value is the difference between the current and the previous value.
+If there is no previous value, the value, the value is the current value and the `start_time` equals `stop_time`.
+
+### TODO
+
+- [ ] Handle errors better: currently if one tenant fails to gather metrics, the whole iteration fails and metrics are not sent for any tenant.
+- [ ] Add retries
+- [ ] Tune the interval